The flexible copper-clad circuit substrates which are used for printed circuit substrates of electronic instruments recently have a serious problem of increase in heat generation accompanied by rise up of package density and wiring pattern density. Thus an important subject is to improve high-temperature stability of the substrates. High-temperature resin which substitutes conventional epoxy resin, includes heat-stable epoxy, polyamide imide, polybenzimidazole, silicone, and polyimide resin. These resins, however, are not always satisfactory to the peel strength to copper foil. Among these resins, polyimide in particular is excellent in high-temperature stability and electrical characteristics, and has increasingly been interested as the materials for electrical insulation. Polyimide, however, has poor bonding strength to metals and requires adhesives to bond the copper foil when polyimide is used for the base film of copper-clad circuit substrates. In spite of good high-temperature stability, insufficient thermal stability of the adhesives inhibits the full utilization of essential high-temperature stability of polyimide. Therefore, polyimide is extensively required which is excellent in high-temperature stability and adhesive strength to metals.
The object of this invention is to provide the high-temperature stable and flexible copper-clad circuit substrates which cause no reduction of adhesive strength during and after use at high temperatures and require no adhesives.